Transparent conductive multi-layer films comprise a support which exhibits high light-transmittance and/or low haze (the degree to which light is scattered as it passes through a material), as well as high insulating properties, overlaid with a thin conductive layer containing an electro-conductive material. Such films must exhibit high surface conductivity while maintaining high optical transparency and have been used as transparent electrodes in the manufacture of photovoltaic cells, EMI shielding screens, flat liquid crystal displays, electroluminescent devices and touch screens in electronic equipment (for instance PDAs, mobile phones etc). Thin film photovoltaic (PV) cells are of particular interest and for this utility the support must exhibit high light transmittance. Very low haze is not a requirement for supports suitable for PV cells, and in fact a significant amount of haze can be beneficial (U.S. Pat. No. 5,078,803; Thin Solid Films 2007, 515, 8695) since it increases the path-length of light as it travels through the PV layer.
The support can be a glass, ceramic or polymeric substrate, and recent developments in flexible electronic devices have focussed on the use of polymeric substrates. Flexible substrates allow manufacture of transparent conductors in a low-cost, high throughput process. Transparent conductive films composite films have typically been produced by vacuum deposition or sputtering techniques. Wet-coating methods have also been used to prepare transparent conductive films, by applying to a substrate a coating composition comprising conductive particles and typically also a binder resin, which is then dried (or sintered) at high-temperature to form a conductive layer, as disclosed in for example U.S. Pat. No. 5,504,133, JP-A-8-199096, JP-A-9-109259, U.S. Pat. No. 5,908,585, U.S. Pat. No. 6,416,818, U.S. Pat. No. 6,777,477, said dried layer may be then subsequently compressed as disclosed in for example in US-2007/0145358 and US-2008/0026204.
Typically, the conductive layer comprises a conductive metal oxide such as indium tin oxide (ITO). However, metal oxide films are fragile and prone to damage during bending or other physical stresses. They also require elevated deposition temperatures and/or high annealing temperatures to achieve high conductivity levels, and this can limits the applicability of vacuum deposition techniques in the manufacture of flexible electronic devices based on polymeric substrates. In addition, vacuum deposition is a costly process and requires specialized equipment, and is not conducive to forming patterns and circuits which typically results in the need for expensive patterning processes such as photolithography. Conductive polymers have also been used as optically transparent electrical conductors, but these generally have lower conductivity values and higher optical absorption (particularly at visible wavelengths) compared to the metal oxide films, and can suffer from lack of chemical and long-term stability.
More recent developments have utilised conductive layers comprising nanowires, as disclosed in, for instance, WO-A-2007/022226, WO-A-2008/046058, WO-A-2008/131304, WO-A-2008/147431 and WO-A-2009/017852. Typically, the nanowires are applied onto a pre-formed substrate and form a surface conductive network. The nanowires must exhibit good adhesion to the substrate and good abrasion resistance. The nanowire network is then over-coated with one or more protective or barrier layer(s), such as an abrasion-resistant or binder layer (e.g. a UV-curable resin layer) in order to impart mechanical integrity or some other characteristic to the conductive layer, while allowing high light-transmittance. It is believed that the network of metal nanowires is partially embedded in the over-coat matrix such that some nanowires may be entirely covered by the matrix while other nanowires may protrude above the surface. Surface conductivity is ensured if enough protruding nanowires percolate above the over-coat matrix.
Thus, the conventional production of transparent conductive films comprising nanowires involves three distinct stages: (i) the preparation of the substrate; (ii) subsequent off-line coating of the nanowires; and (iii) subsequent off-line coating of the protective over-coat layer. Typically, one or both of the off-line coating steps is effected using solvent-coating techniques. It would be desirable to provide a more efficient method of manufacture, for example one which dispenses with the over-coating step, while maintaining the mechanical integrity and abrasion resistance of the conductive layer. In addition, it would be desirable to avoid the use of potentially hazardous and environmentally unfriendly organic solvents in the manufacture of transparent conductive films.